> Like, I need to be able to add shapes to my program, and things I can calculate about shapes to my program, and whenever I do so I need to either add all the

> Like, I need to be able to add shapes to my program, and things I can calculate about shapes to my program, and whenever I do so I need to either add all the new calculations for my new shape, or add my calculation for every existing shape. > That’s the essential complexity at the heart of this problem. Agreed > This is a double dispatch problem no matter what facilities your language has to address it. If your language can pattern match on typed tuples you can use pattern matching to dispatch to the right piece of code. If not, maybe you need to use something like the visitor pattern. But the "double" part refers to the OO solution to that. With pattern matching, there is nothing that sicks out as "double". Pattern matching on two arguments is the same as pattern matching on a single pair. A solution-neutral term might be https://en.wikipedia.org/wiki/Expression_problem https://en.wikipedia.org/wiki/Expression_problem